1. Field of the Invention
The invention relates to a circuit for launching a modulation signal to an optical modulator which, modulates an optical signal, an optical modulator module including a circuit for launching a modulation signal to an optical modulator, and a method of launching a modulation signal to an optical modulator which modulates an optical signal.
2. Description of the Related Art
With significant increase in demand for broad-band multi-media communication services such as Internet, it is now required to develop an optical-fiber communication system having a greater capacity and being able to accomplish higher performance. In addition, with increase in a scale of an optical-fiber communication system, the number of optical communication modules used in an optical-fiber communication system is increasing.
For the reasons mentioned above, a cost and a load of assembly, as well as a size, of an optical communication module used in an optical-fiber communication system are not ignorable in an optical-fiber communication system. Thus, it is now quite important to fabricate an optical communication module in a smaller size, under higher integration of functions, and in smaller costs.
As one of solutions of fabricating an optical-fiber communication system in a smaller size and reducing the number of parts of an optical-fiber communication system, time-multiplicity of data may be increased to increase data-transmission capacity per one wavelength channel. In order to accomplish the solution, an optical communication device associated with high-speed modulation is presently researched and developed.
On the other hand, if data-transmission data per one wavelength channel is increased, wavelength dispersion inherent to an optical fiber path exerts non-ignorable influence on an optical waveform found after long distance transmission. This is because when optical intensity modulation is applied to a light source device, phase modulation (or frequency modulation), though it is quite small, is also applied to the light source device. Such phenomenon is called “wavelength chirping”, and exerts serious influence on long-distance transmission characteristic if a transmission rate is over 2.5 Gb/s per a channel. Hence, an external modulation system in which wavelength chirping is small is mainly applied to an optical-fiber communication system acting as a trunk line. Further developed now for an external modulation system are a single optical-intensity modulator making use of electroabsorption effect of chemical compound semiconductor, a light source including a single optical-intensity modulator and a light source device such as a DFB laser both integrated together in monolithic.
Presently, an optical-fiber communication system having a transmission rate in the range of 2.5 Gb/s and 10 Gb/s per a channel has been already in practice use. There are now developed a ultra-high-speed electroabsorption type optical modulator, an integrated light source used for the optical modulator, and a pigtailed module including them, in order to accomplish a high transmission rate over 40 Gb/s per a channel.
Such an exciter for launching a modulation signal as illustrated in FIG. 7 is often used for integrating an electroabsorption type optical modulator and a modulator-integrated light source. FIG. 7 is a schematic of a conventional circuit 100 for launching a modulation signal to an optical modulator.
The circuit 100 for launching a modulation signal to an optical modulator is comprised of an optical modulator 111, a first stripline 112, a second stripline 113 having a terminator 114, a first bonding wire 115 connecting the optical modulator 111 and the first stripline 112 to each other, and a second boding wire 116 connecting the optical modulator 111 and the second stripline 113 to each other.
The first and second striplines 112 and 113 are arranged sandwiching the optical modulator 111 therebetween in a line in a direction perpendicular to a direction in which an optical signal 131 propagates. The optical signal 131 is coupled into the optical modulator 111, and modulated into a second optical signal 312 in the optical modulator 111 in accordance with a modulation signal 133 output from the first stripline 112.
The circuit 100 can control modulation bandwidth by changing an inductance of the first and second bonding wires 115 and 116. However, the circuit 100 is accompanied with a serious problem that the reflection S11 (see later mentioned FIG. 5) of the circuit 100 is significantly increased in a high-frequency band close to a millimeter wave band and above. This is due to the following two reasons.
The first reason is as follows. A susceptance of the optical modulator 111 acting as if it is a capacitor, when reverse-biased, is equal to zero (open-circuit) in the vicinity of a direct current. However, the susceptance increases up to almost the same level as characteristic admittance of the first and second striplines 112 and 113 in a high band, and hence, turned into a low impedance (a load almost equal to short-circuit).
The second reason is as follows. A load reflecting the second boding wire 116 and subsequent parts becomes a high impedance in the range of a resonance frequency, defined based on both an inductance of the second bonding wire 116 connecting the optical modulator 111 and the second stripline 113 each other, and a capacitance of an optical absorption layer of the optical modulator 111, and a high frequency. Accordingly, the terminator 114 is unlikely to effectively work.
Since an absolute value of the reflection may exceed −10 dB (10% of a modulation RF signal input power), it would be unavoidable that the optical modulator 111 would have much burden, or unnecessary resonance might be found in modulation frequency characteristics.
As a solution to those problems, arrangement of a fixed attenuator in a stage prior to a module for damping a reflection wave to a certain level or below is considered to be the easiest. However, it is difficult to accomplish a broad band and high output both required in such a driver circuit especially for a 40 Gb/s band. Thus, this solution cannot be adopted, because the solution causes a burden with a driver circuit.
For the purpose of improving such impedance mismatching, a strip-line based stub circuit having open or short-circuited ends are often used. However, such stubs can compensate such an impedance mismatch of a load at only a particular frequency, so that it is not suitable for achieving broad-band matching in the range of direct current to millimeter wave. In addition, a stub may not be suitable as such matching circuit which should be deployed in a package having limited dimensions like the optical modulator module from the viewpoints of module design and assembly.
However, since there are no effective solutions other than the above-mentioned solutions, an optical modulator module and a modulator-integrated light source, both of which are still used, though the above-mentioned problems remain unsolved and intensive reflection is not solved.
For instance, Japanese Patent Application Publication No. 2001-209017 has suggested a photoelectric-transfer semiconductor device which carries out impedance-matching in a broad band to provide a high photoelectric frequency. The suggested photoelectric-transfer semiconductor device is comprised of a semiconductor element, a high-frequency electric signal circuit, a circuit for matching resistors, and a circuit for matching capacitors. The semiconductor element carries out photoelectric transfer. The high-frequency electric signal circuit has ends located in the vicinity of the semiconductor element. Among the ends, an end located closest to an electric-signal terminal of the semiconductor element is electrically connected to the electric-signal terminal through an electrical conductor. The circuit for matching resistors is electrically connected at one end to the electric-signal terminal of the semiconductor element through an electrical conductor, and is grounded at the other end. The circuit for matching capacitors is electrically connected to an end of the high-frequency electric signal circuit, and has such an impedance that an impedance at the side of the semiconductor element when viewed from the end is equal to a standardized impedance of the circuit for matching resistors.
Japanese Patent Application Publication No. 2001-154161 has suggested a semiconductor device which allows photo carriers having been generated in the device to sweep out therefrom. The suggested semiconductor device is comprised of a semiconductor element, and a short-circuit element. The semiconductor element is comprised of a semiconductor layer receiving an optical high-frequency signal having a particular frequency, and generating photo carriers, and an output electrode which outputs the photo carriers as high-frequency. The short-circuit element is electrically connected to the output electrode, and keeps the output electrode grounded for the high-frequency.
Japanese Patent Application Publication No. 2000-19473 has suggested a structure of an optical modulator module for using a microstrip line having a small space in which a transmission line is to be fabricated. The suggested structure is comprised of an optical device, a carrier, an optical fiber, a high-frequency terminal, a thermoelectric cooler, a dielectric substrate, and a package. The carrier has electrical conductivity, and the optical device is mounted on the carrier. An optical signal is input into and output from the optical device by the optical fibers. The high-frequency terminal provides an electric high-frequency signal. The thermoelectric cooler keeps the optical device at a constant temperature. A microstrip line is formed on the dielectric substrate. The package holds the above-mentioned parts in it. The package has the high-frequency terminal and a coplanar waveguide. The dielectric substrate is mounted on the carrier. The carrier is exposed at an end thereof closer to the high-frequency terminal. The exposed portion of the carrier, a ground of the coplanar waveguide, the microstrip line, and a signal area of the coplanar waveguide are connected to one another with the bonding wires.
However, the above-mentioned problem of the reflection remains unsolved even in the above-mentioned photoelectric-transfer semiconductor device and the structure of an optical modulator module.
Accordingly, it is an object of the present invention to provide a circuit for launching a modulation signal to an optical modulator, the circuit being capable of suppressing significant increase in reflection particularly in a high-frequency band in which a maximum frequency of a modulation RF signal reaches a millimeter wave band, when an optical modulator and an optical modulator module on which an optical modulator is integrated are modulated at a high-speed.
It is also an object of the present invention to provide a circuit for launching a modulation signal to an optical modulator, which is capable of suppressing significant increase in the above-mentioned reflection without deteriorating a modulation frequency band.
It is further an object of the present invention to provide a circuit for launching a modulation signal to an optical modulator, which is capable of suppressing significant increase in the above-mentioned reflection without necessity of changing circuit elements, parts and a method of fabricating them.
It is further an object of the present invention to provide a circuit for launching a modulation signal to an optical modulator which is most suitable for accomplishing a broader band of an optical modulator module including an optical modulator and a device in which the optical modulator is integrated, driving the optical modulator module at a lower voltage, fabricating the optical modulator module at lower costs, and accomplishing mass production of the optical modulator module.
It is further an object of the present invention to provide an optical modulator module including the above-mentioned circuit for launching a modulation signal to an optical modulator, and a method of launching a modulation signal to an optical modulator, which provides the same advantages as the advantages provided by the above-mentioned circuit for launching a modulation signal to an optical modulator.